In the field of CVD processing, particularly tungsten CVD, it is common to use a perforated gas diffuser plate located above the wafer to be deposited upon, to disperse the reactant gases. One problem which occurs when such apparatus is used, however, is the unwanted deposition of tungsten (or other material) on the gas diffuser plate itself and on other surfaces in the reaction chamber. Particles of the deposited material may fall from the diffuser plate onto wafers during subsequent CVD processing, thereby adversely affecting the end product. Thus, it is imperative to keep the diffuser plate clean from such deposits. One method of accomplishing this is to attempt to prevent the unwanted deposition from occurring at all by precise temperature control of the diffuser plate. Additionally, however, because unwanted deposition typically occurs on other surfaces of the CVD reactor, including the wafer platen (heated susceptor), regardless of the precautions taken, RF plasma cleaning of the CVD reactor frequently must be performed. To accomplish this, it is known to use the gas diffuser plate as an RF electrode.
It is known in the prior art to control the temperature of gas diffuser plates by applying air or liquid cooling to the diffuser plate via welded channels in, or metal tubing on, the diffuser plate. Alternatively, heat may be conducted away indirectly via air- or liquid-cooled surfaces upon which the diffuser plate rests. In either case, preventing unwanted deposition on the gas diffuser plate by means of temperature control presents several difficulties, including: 1) the high rate of radiant energy flow from wafer to gas diffuser plate; 2) obtaining temperature control without disturbing the gas flow pattern through the gas diffuser plate; 3) obtaining temperature control without introducing a cooling fluid to the interior of the reactor process chamber, either in tubes or welded channels; 4) obtaining symmetrical cooling of the gas diffuser plate; and 5) obtaining a sufficient rate of cooling to control the gas diffuser plate temperature within acceptable limits.
When the gas diffuser plate is used as an RF electrode for RF plasma cleaning, as discussed above, the cooling tubes on the diffuser plate are used as RF conductors and must be electrically isolated from the rest of the reactor. Alternatively, an isolated power tap passes through the wall of the reactor chamber and is attached to the isolated gas diffuser plate. In the latter case, the cooling tubes must be electrically isolated from the rest of the reactor. Several difficulties arise when introducing RF power to a temperature-controlled gas diffuser plate for the purpose of RF cleaning the heated wafer susceptor and the gas diffuser plate itself. These include the following: 1) insulating the rest of the reactor from the RF power on the gas diffuser plate; 2) positioning the RF power input to obtain even power distribution on the gas diffuser plate without affecting the temperature control system; and 3) providing RF power input without significant RF power loss due to electrical resistance in the input.
The present invention is designed to overcome the various drawbacks noted above with respect to gas diffuser plate temperature control and RF plasma cleaning apparatus.